One of the most commonly seeded crops in Canada is canola, a cultivar of oilseed rape (Brassica napus). As a mass&#8208;flowering crop grown intensively throughout the Canadian Prairies, canola has the potential to influence pollinator success across tens of thousands of square kilometers of cropland. Bumble bees (Bombus sp.) are efficient pollinators of many types of native and crop plants. We measured the influence of this mass&#8208;flowering crop on the abundance and phenology of bumble bees, and on another species of social bee (a sweat bee; Halictus rubicundus), by continuously deploying traps at different levels of canola cultivation intensity, spanning the start and end of canola bloom. Queen bumble bees were more abundant in areas with more canola cover, indicating that this crop is attractive to queens. However, bumble bee workers were significantly fewer in these locations later in the season, suggesting reduced colony success. The median collection dates of workers of three bumble bee species were earlier near canola fields, suggesting a dynamic response of colonies to the increased floral resources. Different species experienced this shift to different extents. The sweat bee was not affected by canola cultivation intensity. Our findings suggest that mass&#8208;flowering crops such as canola are attractive to bumble bee queens and therefore may lead to higher rates of colony establishment, but also that colonies established near this crop may be less successful. We propose that the effect on bumble bees can be mitigated by spacing the crop more evenly with respect to alternate floral resources.

ece32856-fig-0002: Within‐season changes in bumble bee abundance in relation to canola crops for three selected species using separate generalized linear‐mixed models for queens and workers. Interaction plots demonstrate a statistically significant interaction effect. 0% and 40% curves show the predicted interaction with canola cover at the average values for canola‐absent and canola‐present sites, respectively. Points indicate trapping rates at 184 trapping events and are shown as jittered semitransparent circles to reduce overplotting

Mentions:
Worker bumble bees had reduced abundance where canola cover was higher. This effect was observed later in the season, reflecting an interaction between canola cover and Julian day (Figure 2; Table 1). In contrast to workers, queen bumble bees showed significantly higher abundance with higher canola cover, suggesting a positive response to the increased forage availability this represents (Figure 2; Table 1). However, there was an interaction driven by B. borealis where late‐season queens were less abundant in higher canola cover, mirroring the pattern seen in workers. The selected bumble bee models both included species random intercepts and slopes, and these had standard deviations significantly different from zero (Table 1), underlining the importance of accounting for interspecific differences in bumble bee phenology. The abundance of H. rubicundus was not affected by canola cover and showed a peak in abundance early in the season (Figure 3; Table S3).

ece32856-fig-0002: Within‐season changes in bumble bee abundance in relation to canola crops for three selected species using separate generalized linear‐mixed models for queens and workers. Interaction plots demonstrate a statistically significant interaction effect. 0% and 40% curves show the predicted interaction with canola cover at the average values for canola‐absent and canola‐present sites, respectively. Points indicate trapping rates at 184 trapping events and are shown as jittered semitransparent circles to reduce overplotting

Mentions:
Worker bumble bees had reduced abundance where canola cover was higher. This effect was observed later in the season, reflecting an interaction between canola cover and Julian day (Figure 2; Table 1). In contrast to workers, queen bumble bees showed significantly higher abundance with higher canola cover, suggesting a positive response to the increased forage availability this represents (Figure 2; Table 1). However, there was an interaction driven by B. borealis where late‐season queens were less abundant in higher canola cover, mirroring the pattern seen in workers. The selected bumble bee models both included species random intercepts and slopes, and these had standard deviations significantly different from zero (Table 1), underlining the importance of accounting for interspecific differences in bumble bee phenology. The abundance of H. rubicundus was not affected by canola cover and showed a peak in abundance early in the season (Figure 3; Table S3).

One of the most commonly seeded crops in Canada is canola, a cultivar of oilseed rape (Brassica napus). As a mass&#8208;flowering crop grown intensively throughout the Canadian Prairies, canola has the potential to influence pollinator success across tens of thousands of square kilometers of cropland. Bumble bees (Bombus sp.) are efficient pollinators of many types of native and crop plants. We measured the influence of this mass&#8208;flowering crop on the abundance and phenology of bumble bees, and on another species of social bee (a sweat bee; Halictus rubicundus), by continuously deploying traps at different levels of canola cultivation intensity, spanning the start and end of canola bloom. Queen bumble bees were more abundant in areas with more canola cover, indicating that this crop is attractive to queens. However, bumble bee workers were significantly fewer in these locations later in the season, suggesting reduced colony success. The median collection dates of workers of three bumble bee species were earlier near canola fields, suggesting a dynamic response of colonies to the increased floral resources. Different species experienced this shift to different extents. The sweat bee was not affected by canola cultivation intensity. Our findings suggest that mass&#8208;flowering crops such as canola are attractive to bumble bee queens and therefore may lead to higher rates of colony establishment, but also that colonies established near this crop may be less successful. We propose that the effect on bumble bees can be mitigated by spacing the crop more evenly with respect to alternate floral resources.